The ras oncogene is involved in the formation of a significant portion of human tumors. Ras, the protein encoded by ras, is a small guanine nucleotide-binding protein which inappropriately signals cell growth in its oncogenically activated form. Unpublished studies using pharmacological inhibitors of cellular Ras activity (farnesyl transferase inhibitors, or FTIs) have revealed that a Rho family protein, RhoB, may be necessary for malignant cell transformation by oncogenic ras. Rho proteins are part of the Ras superfamily of guanine nucleotide-binding proteins and have been implicated in regulation of specific actin cytoskeletal structures. Some Rho proteins have been speculated to be involved in Ras-induced signaling and/or cytoskeletal changes, but this question has not been directly tested; RhoB itself is biologically uncharacterized and its molecular effector functions are unexplored. An identification of the activities and effector molecules controlled by RhoB signaling is important because of (1) the potential therapeutic importance of these functions and (2) their likelihood of providing basic insights into our understanding of cytoskeleton-based signal transduction. Significantly, FTIs cause reversion of cells transformed by oncogenic Ras but have little effect on the growth of normal cells, despite their requirement for normal Ras activity. Because we have determined that a critical event in FTI-induced reversion of ras- transformed cells process is inhibition of Rhob, the results suggest that malignant cell growth may preferentially require a RhoB-dependent function(s) which may be largely dispensible in normal cells. Underlying the clinical relevance of this issue is the more fundamental question of how cytoskeletal actin structures may function in signal transduction, the current understanding of which is heavily weighed toward the study of diffusible signaling agents. It would therefore be important to identify the molecular effector molecules and downstream signaling targets of RhoB, in order to begin to dissect pathways that could be mechanistically novel as well as well as therapeutically useful. We propose to address these issues in a test of the hypothesis that RhoB- regulated processes are critical for Ras-dependent signal transduction and malignant cell transformation. Briefly, we aim to: genetically inhibit cellular RhoB activity to establish that it is necessary for initiating and maintaining ras-dependent cell transformation; identify ras- and/or growth factor-activated "immediate-early" genes and regulatory elements which are controlled by RhoB signaling activity; clone RhoB effector molecule(s); genetically inhibit candidate effector molecules in normal or ras- transformed cells to establish that their activity is necessary for normal or transformed cell growth